To determine the leukemogenic effect of a heavy ion beam, male RF/J mice were irradiated with fully stripped carbon ions from the Bevatron. Only the plateau portion of the beam was used, with beam energy of 220 MeV/amu when entering and 115 MeV/amu when leaving the mice. Corresponding mean LET values at entrance and exit were calculated to be 14 keV/micron and 19 keV/micron respectively. One hundred thirty mice were exposed to 65 rad, and 100 mice were exposed to 130 rad from the heavy ion beam, and matching groups were exposed to Co gamma rays. At 15 months after irradiation, overall mortalities have reached 30 percent for the 65 rad groups and 65 percent for the 130 rad groups. The mortality curves for the carbon and gamma irradiated groups are identical. While the histopathologic examinations have not been completed, the great majority of mice to-date died with thymic lymphoma.